Ultrahigh Damping Capacities in Lightweight Structural Materials

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Ultrahigh Damping Capacities in Lightweight Structural Materials Andrea Knöller, Stefan Kilper, Achim Diem, Marc Widenmeyer, Tomce Runcevski, Robert E. Dinnebier, Joachim Bill, and Zaklina Burghard Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00194 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Ultrahigh Damping Capacities in Lightweight Structural Materials Andrea Knöller1, Stefan Kilper1, Achim M. Diem1, Marc Widenmeyer1, Tomče Runčevski2,3, Robert E. Dinnebier4, Joachim Bill1, Zaklina Burghard1,* 1

Institute for Materials Science, University of Stuttgart, Heisenbergstr. 3, 70569 Stuttgart, Germany

2

Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, USA 3

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 4

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

The demand to outperform current technologies pushes scientists to develop novel strategies to enable fabricating materials with exceptional properties. Along this line, lightweight structural materials are of great interest due to their versatile applicability as sensors, catalysts, battery electrodes and acoustic or mechanical dampers. Here we report a strategy to design ultralight (ρ = 3 mg/cm3) and hierarchically structured ceramic scaffolds of macroscopic size. Such scaffolds exhibit mechanical reversibility comparable to that of microscopic metamaterials, leading to a macroscopically remarkable dynamic mechanical performance. Upon mechanical

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loading, these scaffolds show a deformation mechanism similar to polyurethane (PU) foams and this resilience yields ultrahigh damping capacities tan δ of up to 0.47.

Keywords: Vanadium pentoxide, lightweight materials, ice-templating, mechanical damping

Developing novel lightweight materials encourages their application in areas including sensing, catalysis, energy storage and mechanical damping. For such applications, porous, yet mechanically robust materials can be realized through a hierarchical organization of their constituents.1,2 Such synthetic structural materials exhibit enhanced mechanical stability, in which the relative Young’s Modulus and the relative density typically exhibit a quadratic relationship (E/Es = (ρ/ρs)2),3 in contrast to less ordered structures (E/Es = (ρ/ρs)3).1 Besides the structuring, the choice of building blocks also plays a major role for the desired field of application. For example, vanadium pentoxide is a well-known candidate for most of the above mentioned applications,4–7 and can be also fabricated in the form of porous scaffolds.8,9 However, its brittle ceramic nature typically keeps it from application as a shock- or vibration resistant mechanical damper. The ceramic’s brittleness can be overcome by downscaling the structural features to the nanometer range, which unlocks mechanical properties that strongly differ from the ones of their bulk equivalents.10 For instance, small sizes and high aspect ratios induce a pronounced mechanical flexibility in strut- and fiber-like geometries,11 such as V2O5 nanofibers.12,13 Introducing mechanical resilience to a ceramic material would not only increase the material’s functional lifetime, when being exposed to vibrational impact, but also open new fields of applications.

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The fabrication strategy to obtain ultrahigh damping ceramic scaffolds involves the instant freezing of aqueous V2O5 suspensions with liquid nitrogen (Figure 1a). The sol-gel derived V2O5 building blocks used here are in the form of hydrated nanofibers [(h)V2O5] with a rectangular shape and lengths of several micrometers.12,13 Moreover, they exhibit a pronounced, dimensiondependent mechanical flexibility.11,13 During the freezing, ice crystal platelets form and serve as a structural template14 for the emerging material with a centrosymmetric microstructure.15,16 Subsequent freeze drying results in self-supporting scaffolds with a water content of 13.47 wt%, as derived from thermogravimetric analysis (TGA) (Supporting Figure S1). In order to obtain allceramic scaffolds, thus to remove the residual water, the scaffolds were subsequently annealed at 350 °C in air (Figure 1a), as suggested by TGA. This thermal treatment not only removes the excess water, but also has an impact on the crystal structure and morphology of the nanofibers. A color change from red-brown15 to bright yellow (Figure 1b) indicated that the scaffolds underwent a phase change. Differential scanning calorimetry (DSC) revealed two exothermal phase transitions at 295 °C and 339 °C peak temperature, respectively (Supporting Figure S1). The latter represents a transition from an amorphous phase to a crystalline one. This recrystallization was also detected by X-ray powder diffraction (XRPD) (Supporting Figure S2).

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Figure 1: From single V2O5 nanofibers to highly ordered all-ceramic scaffolds. (a) Schematic depiction of scaffold formation. V2O5 nanofibers are randomly oriented in aqueous suspension. During instant freezing with liquid nitrogen, lamellar ice crystals grow inside the suspension, compacting the nanofibers in-between. This structural arrangement is preserved after freeze drying and thermal treatment, leading to highly organized, self-supporting V2O5 scaffolds. (b) Photograph of a cylindrical V2O5 scaffold with a diameter and height of ~8 mm. SEM images of (c) the scaffold’s lamellar microstructure and (d) the filigree nanofiber assembly forming the nanometer thick lamellas and pillars. Owing to the fabrication procedure, the lamellas are oriented in a centrosymmetric manner.

Based on the nanofiber concentration of 3.55 mg/ml, the resulting all-ceramic scaffolds exhibit an ultralight weight with a density of 3 mg/cm3, which is also reflected in the highly permeable

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microstructure, as revealed by scanning electron microscopy (SEM). In particular, the resulting V2O5 scaffolds have a lamellar microstructure (Figure 1c), which can be also found in several other ice-templated ceramic scaffolds.17–19 Moreover, the lamellas of the V2O5 scaffolds are interconnected by numerous pillars. Closer inspection of the lamellas and pillars reveals that the initially single nanofibers had been fused together, forming V2O5 fibrous bundles with thicknesses of several tens of nanometers (Figure 1d). The mechanical properties of the V2O5 scaffolds were investigated by performing compression tests. Figure 2a presents the stress-compressive strain curve of a scaffold under quasi-static loading. This curve shows a typical progression for cellular solids with their three characteristic regimes: (I) linear elastic deformation, (II) non-linear deformation and (III) densification.3 Extracted from the linear slope in regime I, plotting the relative Young’s modulus E/Es over the relative density ρ/ρs (with E and ρ as Young’s modulus and density of the scaffolds, respectively. Es and ρs are the Young’s modulus20 and density of the scaffolds’ walls) in figure 2b shows that the scaffolds are located on the dashed line, which embodies the quadratic relationship E/Es = (ρ/ρs)2. This relationship is also found in other highly porous and hierarchically structured materials,1,15,21 which show a superior mechanical performance compared to highly porous aerogels with a random microstructure.22,23 Regime II (Figure 2a) exhibits a more levelled slope, implying non-linear deformation mechanisms, which are partially reversible, as illustrated by the progression of the curve after releasing the mechanical load. A reversible deformation of 3 % (between 50 and 47 % compressive strain in regime III) can be detected. This value is unusual for all-ceramic materials, as they are known to be stiff and brittle, and prompted investigation of the V2O5 scaffolds in terms of their dynamic mechanical behavior.

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Figure 2: Mechanical stability of the V2O5 scaffolds. (a) Stress-compressive strain curve of a V2O5 scaffold up to 50 % compressive strain, revealing three regimes: (I) linear elastic deformation, (II) plateau at which irreversible deformation occurs, and (III) beginning densification. (b) Relative Young’s modulus of different highly porous materials plotted over their relative density. The Ni-P microlattices,1 the Al2O3 nanolattices21 and the V2O5 scaffolds15 exhibit ordered microstructures, while the SiO222,23 and Al2O3 aerogels23 exhibit random microstructures.

Hence, subsequent to initial loading, the scaffolds were dynamically loaded for 100 cycles over a range of 3 % compressive strain. Figure 3a shows the stress-compressive strain curve of the corresponding cyclic compression test. As the cycles overlap, the insert of figure 3a uncovers

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the exact evolution of the cycles. It can be seen that the loops become narrower and steadily decrease in stress, which is reflected in the slightly decreasing slope (Figure 3b). This slight degradation is most likely attributable to irreversible structural changes upon cycling. A comparable reversible mechanical deformation has so far been reported only for a limited number of ceramic scaffolds, so-called structural metamaterials.2,21,24 Such metamaterials feature periodically arranged hollow ceramic struts with wall thicknesses in the nanometer range, leading to three-dimensional nanolattices. TiN nanolattices with a wall thickness of 80 nm exhibit a partial reversible deformation of about 1 % to 1.5 % (in 3 sets of 11 loading-unloading cycles).2 These values are about half of the reversible compression the V2O5 scaffolds are capable of. Furthermore, the compression cycles of the TiN scaffolds showed a much more pronounced residual displacement after each cycle, which is attributed to serious permanent deformation due to nanocracking. Such a pronounced off-set was not detected in the case of the V2O5 scaffolds, due to the size and geometry of the V2O5 nanofibers. They display a noticeable mechanical flexibility, characteristic for ultrafine ceramic fibers.11 This flexibility is preserved when the nanofibers are assembled into the filigree structured V2O5 scaffolds, making them far less rigid and brittle than the hollow-tube lattice members within the TiN nanolattices. Meza et al.21 recently investigated the impact of wall thickness and relative density on the mechanical performance of Al2O3 nanolattices. They reported a significant improvement in reversibility, as the wall thickness and relative density are decreasing. However, their fabrication method limits the sample size (< 100 µm), and thus their application to the microscopic level. In contrast, our approach allows generating macroscopic structural ceramics (> several mm, Figure 1b) with pronounced reversibility, expanding the possible field of application.

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Figure 3: Mechanical flexibility of the V2O5 scaffolds. (a) Stress-compressive strain curve of a V2O5 scaffold under cycling loading. Displayed are 100 overlapping cycles. The insert represents cycles 1, 10, and 100, visualizing the progression of deformation. (b) Corresponding slope over all measured cycles. (c) SEM cross-sectional images of a V2O5 scaffold, which is mechanically deformed by micromanipulators perpendicular as well as parallel to the lamella orientation. After removing the applied mechanical load, the scaffold returns to its initial microstructure.

The pronounced reversible deformation of the V2O5 scaffolds was microscopically visualized by in situ mechanical deformation performed using an SEM equipped with two micromanipulators. As the scaffolds macroscopically exhibit a centrosymmetric lamella orientation within the cylindrical samples,15 the two limit cases (lamella orientation

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perpendicular and parallel to the direction of compression) were independently investigated (Figure 3c). Expectedly, by applying a mechanical load in both directions, the scaffolds’ walls deform elastically. Compressing the scaffold perpendicular to the lamellas resulted in bending of the bridging pillars, which themselves straightened upon releasing the load (Supporting Movie S1). Applying a compressive load parallel to the lamellas led to buckling of the lamellas, which was likewise reversible (Supporting Movie S2). Even though the V2O5 scaffolds consist only of ceramic matter, they do not mechanically deform like other common ceramic scaffolds. Ashby summarized the mechanical deformation of different cellular solids.3 He showed that ceramic scaffolds deform irreversibly due to brittle crushing, while polymeric foams, such as PU, show elastic cell wall buckling. Based on the in situ SEM observations, the V2O5 scaffolds behave more like polymeric than conventional ceramic materials. Thus, the flexibility of the V2O5 building blocks was transferred to the macroscopic level.

In order to explore the V2O5 scaffolds’ full dynamic mechanical potential, compressive strainand frequency-dependent damping tests were conducted (Supporting Figure S3). The colorscaled 2D maps of storage and loss modulus are given in figure 4. At a constant frequency, the storage modulus increases until 10 % compressive strain, and then drastically decreases for higher compressive strains. This trend is in good agreement with the V2O5 scaffolds’ static compressive behavior (Figure 2a), which shows a linear trend until 10 % (regime I). In this regime the scaffold walls become stiffer under elastic compression, causing the storage modulus to increase correspondingly.25 Cell wall buckling, similar to what is found in PU foams3 (Supporting Figure S4) initially reduces the storage modulus, when exceeding 10 % compressive strain. The loss modulus reaches its maximum at compressive strains higher than 10 %, at which

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the scaffolds deform irreversibly. This deformation is accompanied by friction between neighboring lamellas, as they contact each other, and breakage of the structural features. Hence, in this regime the energy dissipation becomes dominant. In contrast, analyzing the frequency dependency at a fixed compressive strain reveals that both storage and loss modulus gradually increase with increasing frequency. For higher frequencies, the scaffolds cannot instantly follow the applied oscillation, thus they appear to be stiffer. In fact, the enhanced stiffness results in an increased Young’s modulus, which similarly affects the storage modulus.25 In addition, as the material’s response is lagging behind larger phase angles, thus generating larger hysteresis, the loss modulus also clearly increases. The generated data of storage and loss modulus enable calculation of the loss factor tan δ (= loss modulus/storage modulus), which is a measure of a material’s damping capacity (Figure 4c). The tan δ of the V2O5 scaffolds show strong compressive strain- and frequency dependencies, resulting in a maximum of 0.47 for the highest measured compressive strain and frequency. Therefore, these dependencies allow actively tuning the viscoelastic properties of the V2O5 scaffolds. As there has only been limited work done in the field of damping capacities of ceramic materials30 and of damping ceramic scaffolds in particular, the results here reported were compared with damping capacities of foams and scaffolds of other material classes.1,25–29 Figure 4d displays the loss factors of polymeric, metallic, ceramic and composite foams/scaffolds. PU foams and Al-PU foams exhibit higher damping capacities than most reported metal foams. This superiority is mostly attributed to the intrinsic material damping of the soft polymer.31 Metals and ceramics, on the other hand, usually exhibit poor damping behaviour,30 which is especially reflected in the low values of the Mg alloy/SiCP foam reported

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by Huang et al.29 Even though these foams are highly porous, thus allowing air damping,32 their microstructure with its thick and dense cell walls prevents such foams from showing pronounced damping effects. In contrast, the V2O5 scaffolds reported here achieve ultrahigh damping capacities, which are attributed to the flexibility of the nanofibers coupled with their filigree, hierarchical arrangement.

Figure 4: Compressive strain- and frequency dependent viscoelastic response of the V2O5 scaffolds. (a) Storage modulus, (b) loss modulus, and (c) loss factor tan δ. (d) Damping capacities of different types of foams. Loss factors tan δ of polymer-, metal-, ceramic- as well as composite foams/scaffolds.1,25–29

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These findings are valuable for the design of structural materials made from low damping materials, such as metals and ceramics, providing useful information to help develop further macroscopic scaffolds with remarkable dynamic mechanical behavior.

ASSOCIATED CONTENT The supporting information is available and free of charge. Materials and methods TGA and DSC (Figure S1) XRPD Pattern (Figure S2) Description of the Damping test method (Figure S3) Damping maps of PU foams (Figure S4) SEM in situ mechanical investigation (Movies S1 and S2)

AUTHOR INFORMATION Corresponding Author *Zaklina Burghard (Correspondence to: [email protected]) Author Contributions A.K. conceived the idea to this work, prepared all samples, conducted SEM investigations and mechanical testing. A.K. and S.K. customized the damping test method. A.K. and A.M.D. conducted the in situ SEM investigations. M.W. performed TGA/DSC measurements. R.E.D. provided the XRPD measurements and analyzed the data in cooperation with T.R., who was also

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involved in the discussion about this work. Z.B. and J.B. supervised the work. A.K. and Z.B. prepared the manuscript with contributions from all co-authors. Funding Sources DFG (BI 469/17-2 and BU 2713/2-1), Baden-Württemberg Stiftung, the International MaxPlanck Research School for Condensed Matter Science and the Landesgraduiertenförderung Acknowledgments The authors thank B. Fenk for installation of the micromanipulators and technical assistance, F. Adams for XRPD measurements, H. Pfaff for providing assistance with customizing the damping test method, T. Jahnke for fruitful discussions, R. Segar for proofreading as well as the scientific facility Nanostrukturlabor of J. Weis and the department of J. Spatz from the Max Planck Institutes in Stuttgart, Germany, for their technical support and equipment access. Financial support by the DFG (BI 469/17-2 and BU 2713/2-1), the Baden-Württemberg Stiftung, the International Max-Planck Research School for Condensed Matter Science and the Landesgraduiertenförderung is greatly appreciated.

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